Description of Related Art
The bandgap of III-nitride materials, including (Al, Ga, In)—N and their alloys, extends from the very narrow gap of InN (0.7 eV) to the very wide gap of AlN (6.2 eV), making III-nitride materials highly suitable for optoelectronic applications such as light emitting diodes (LEDs), laser diodes, optical modulators, and detectors over a wide spectral range extending from the near infrared to the deep ultraviolet. Visible light LEDs and lasers can be obtained using InGaN in the active layers, while ultraviolet (UV) LEDs and lasers require the larger bandgap of AlGaN.
Visible spectrum LEDs based on InGaN and AlInGaP systems have reached maturity and are now in mass production. However, the development of UV LEDs is still hampered by a number of difficulties involving basic material properties of AlGaN alloys, especially those with high Al content. Compared to LEDs in the visible spectral range with external quantum efficiency (EQE, the ratio of extracted photons to injected electron-hole pairs) of more than 50%, deep UV LEDs, such as those emitting below 300 nm, have an EQE of only up to 1%.
UV LEDs with emission wavelengths in the range of 230-350 nm are expected to find a wide range of applications, most of which are based on the interaction between UV radiation and biological material [Khan et al., 2008]. Typical applications include surface sterilization, water purification, medical devices and biochemistry, light sources for ultra-high density optical recording, white lighting, fluorescence analysis, sensing, and zero-emission automobiles. Although under extensive research for many years, UV LEDs, especially those emitting below 300 nm, remain extremely inefficient when compared to their blue and green counterparts. For example, Hirayama et al. recently reported 10.5 mW single-chip LED operation at 282 nm and peak EQE of 1.2% [Hirayama et al., 2009].
Poor current spreading has been one of the major stumbling blocks to obtaining high efficiency deep UV LEDs, due to difficulties in achieving highly conductive yet sufficiently thick n-type AlGaN bottom cladding layers with high Al content. In 2004, Adivarahan et al. proposed a “micro-pixel” LED. The device consists of a 10×10 micro-pixel LED array, with each pixel being a circular mesa of diameter 26 μm. The total physical dimension of the device is 500 μm×500 μm. Since the lateral distance for electron migration before its recombination with a hole is significantly reduced using such geometry, the differential resistance of the device is lowered to 9.8Ω, as compared to standard square geometry LEDs based on the same epitaxial layers with differential resistances from 40 to 14.4Ω [Adivarahan et al., 2004]. Also in 2004, Kim et al. investigated the trade-off between mesa size and output power of circular-geometry deep UV LEDs, and found that without obtaining more conductive n-type and p-type AlGaN cladding layers, the optimized diameter for circular-disk deep UV LED is limited to about 250 μm [Kim et al., 2004].